Multiband radio receiver



Feb. 4, 1941. K VQ L M 2,230,554

MULTIBAND RADIO RECEIVER Filed June 3, 1938 I 50 200 300 4 00 5 00 6 00 I030 1.5 00 2103 KC 2000 750 600500 200 METERS INVENTOR. KARL W/L HELM A T-TORNEY.

i atented Feb. 4, 1941 UNITED STATES PATENT OFFICE MULTIBAND RADIO RECEIVER Karl Wilhelm, Berlin, Germany, assignor to Telefunken Gesellschaft fiirDrahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Ger- Radio receivers are sometimes furnished with a tunable band-pass filter, such as, an input band-pass filter, in order that even in advance of the first or input tube such marked and sharp filtering may be obtained that cross modulation in the first tube is avoided. In adapting such receivers for short-wave reception the first oscillatory circuit of the band-pass filter need not be and is not utilized because here there is no risk of cross modulation in View of the low field intensity and since unnecessary attenuation of the incoming signals by a band-pass filter would be occasioned. Such receivers therefore are uneconomical by reason of the fact that the rotary condenser of the first or input circuit is not utilized in short-wave reception.

Now, in the invention which is concerned with a receiver of this kind, the said drawback is avoided. According to the invention the rotary condenser of the band-pass filter is unused in short-wave reception and forms conjointly with a short-wave coil the oscillation circuit of a tunable stage which in broadcast reception is aperiodic. In a receiver of the superheterodyne type wherein the first or input tube is a radio frequency amplifier, it will thus be feasible to use only two rotary condensers in lieu of three for the input circuits in that the plate circuit of the input tube for broadcast wave reception including long-wave band is made aperiodic in nature. It will be understood that for short-wave reception the said aperiodic coupling, because of the presence of disturbing parallel capacitances, is no longer practicable. Therefore the rotary condenser of the input band-pass filter which is unused in short-wave reception, together with a short-wave coil is cut in the plate circuit of the input tube making it possible to obtain additional gain which is so much desired in short-wave reception. Hence, it is possible only in this manner to use aperiodic coupling for broadcast wave reception.

Fig. 1 shows an exemplified embodiment of the invention, The input band-pass filter comprises the two oscillation circuits I and II. Included in the plate circuit of tube R functioning as a radio frequency amplifier is an ohmic resistance W, and in parallel relationship thereto is provided a short-circuit path Z designed to preclude incoming frequency of the same value as the I. F. when the invention is applied to a receiver of the superheterodyne type, in which case the output of amplifier R is fed to a conventional mixer or converter circuit. Otherwise, as in the case of a receiver of the tuned radio frequency type,

the output of amplifier R will be fed to a detector or to another radio frequency amplifier. The switches (which bear no special designations) serve to effect the change over from broadcast to long-wave reception. In case of broadcast wave reception they are closed as shown. The coupling of the two oscillation circuits is effected by way of the coupling coils 8'1 and S'z.

For short-wave reception the antenna is associated with the short-wave coil K by way of the switch S1, the said coil being at the same time connected in parallel relation to the rotary condenser C2 by closure of the switch S2 and to the coils for the other wave bands. The switch S3 at the same time is opened and the switch S4 is closed, with the result that the rotary condenser C1 is removed from the oscillation circuit and instead connected in parallel to the shortwave coil K included in the plate circuit of the tube R, the said coil, if desired, remaining in the plate circuit also for the other wave-bands.

Finally, also the switch S5 is closed in order to render resistance W inoperative.

The fact that the I. F. short-circuit path (bypass) Z and the resistance W are connected in parallel has still another significance. For when the I. F. is between the broadcast and the longwave bands, say, at around 460 kc. (equal to around 640 meters), the I. F. circuit acts, for the shorter waves, as an inductance which, for a wave falling inside the medium-Wave band comes to be in resonance with the spurious parallel capacitance of resistance W. By suitable choice of the relations of the coil and the condenser of circuit Z this resonance may be shifted on a wave, say, 500 meters, so that here maximum gain will be brought about. This is favorable in so far as inside this range fall most of the stations coming into consideration, In Fig. 2 this resonance crest is clearly visible. In fact, without this circuit organization and without dimensions as here to be chosen, the gain would follow a shape as roughly indicated by the dash-line. It will be noticed that for waves of around 750 meters there occurs a marked decrease for the reason that for these waves the short-circuit path acts like a capacity. However, this offers no disadvantage since no important transmitter stations fall inside this part of the band.

What I claim is:

k 1. In a multiband receiver, an antenna having an input coil, a vacuum tube amplifier having a tuned input circuit and an aperiodic output circuit, a tunable band pass filter including a condenser intercoupling the antenna coil and the including a variable condenser, and switching means which in the position for reception of a long Wave band connects the band pass filter in coupling relation between the antenna circuit and the tunable input circuit, and in the position for reception of a short wave band couples the antenna circuit directly to the tunable input circuit and simultaneously connects the variable condenser of the band pass filter in the output circuit.

KARL WILHELM. 

